Ironing the wrinkles out of copper tubing

How a Wrinkle Forms

Simply put, as a tube transitions from a straight section to a bent section, it undergoes compressive stresses on the inside of the bend. When these compressive stresses exceed a certain limit, the inside wall may buckle. The result is a protrusion that forms away from the wiper die and toward the outside wall of the tube (because the material takes the path of least resistance). A wiper die is used to help offset or limit these compressive stresses in an attempt to make a wrinkle-free bend. So the question is, How exactly does a wiper die limit the compressive stresses when attempting a tight-radius bend?

First, in a way similar to how pinch tooling works to improve the outside appearance of a bent tube, a wiper die helps support a tube as the pressure die first makes contact with the tube. Second, once the pressure die is in contact with the tube, the wiper die continues to apply a force on the tube while the bending process is under way. Throughout the bending cycle, the wiper die is critical to supporting the intrados, or inside bend, of the tube.

Right before the tube experiences a bending torque, the wiper die goes to work, preventing the tube from separating from the pressure die. Figure 1 depicts a tube separating from the pressure die without the aid of a wiper die. This can be most obvious when bending a soft material such as 3003 aluminum or copper. The condition usually arises when the centerline bending radius approaches the tube's OD (1D bend).

If the tube heels away significantly from the pressure die before or during the bending motion, the tube may undergo an additional compressive stress component in the area surrounding the bend tangent. Thus, one goal of a wiper die is to ensure the tube stays aligned (parallel) with the pressure die as the tube is being drawn and bent.

The Bridge Support Concept

If you have a grip section in a bend die opposing the pressure die, and you have the wiper die on the other side of bend tangent also opposing the pressure die, then the pressure die is supported on both sides (see Figure 2).

A wiper die imparts force on the tube before and during the bending motion; this force counteracts the compressive forces within the tube induced by the bending process.